In recent years, with progress in digital technologies and improvement in data compression technologies, the optical disk such as a CD (Compact Disk) and a DVD (Digital Versatile Disk) enabled to record an amount of data corresponding to approximately 7 times that of the CD in a disk having the same diameter as the CD, etc., has been watched with keen interest as a medium for recording user data such as music, movies, photographs, and computer software, and, with its price decreasing, an optical-disk apparatus having the optical disk as a target medium for recording the data has become commonplace.
In the optical disk, information is recorded by means of each of the lengths of a mark area and a space area and combinations thereof, each of the areas having reflectance different from each other. Thus, when recording information in the optical disk, the power of a laser beam output from a light-beam source (below also referred to as “the light-emission power”) is controlled such that each of the mark areas and the space areas is formed at a predetermined location.
In a recordable-type optical disk such as a CD-R (CD-Recordable), a DVD-R (DVD-Recordable), and a DVD+R (DVD+Recordable), etc., containing on a recording surface organic colorants, when forming the mark area, the light-emission power is increased so as to heat and dissolve the colorants, deforming/transforming a portion of a disk substrate contacting therewith. On the other hand, when forming the space area, the light-emission power is decreased to about the same as when reproducing so that the disk substrate does not deform/transform. Hereby, the reflectance is lower in the mark area than in the space area. Such method of controlling the light-emission power is also called a single-pulse recording method. It is to be noted that the light-emission power when forming the mark area is also called the write power and the light-emission power when forming the space area is also called the bottom power.
Moreover, in a rewritable optical disk such as a CD-RW (CD-ReWritable), a DVD-RW (DVD-ReWritable), and a DVD+RW (DVD+ReWritable), etc., containing special alloys on a recording surface, when forming the mark area, the special alloys are heated to a first temperature and then rapidly cooled for creating an amorphous state. On the other hand, when forming the space area, the special alloys are heated to a second temperature (lower than the first temperature) and then slowly cooled for creating a crystal state. Hereby, the reflectance is lower in the mark area than in the space area. In this case, the light-emission power when forming the mark area is divided into multiple pulses (multi-pulsed), in order to eliminate an effect of heat accumulation. Such method of controlling the light-emission power is also called a multi-pulse recording method. The maximum value of the multi-pulsed light-emission power is also called the peak power and the minimum value is also called the bottom power. Moreover, the light-emission power for forming the space area is also called the erase power (smaller than the peak power and larger than the bottom power). It is to be noted that, with an increased rate of recording, the multi-pulse recording method is proposed, even for a colorant-type disk, for example a DVD-type optical disk (DVD-R, DVD+R, etc.).
In the recordable-type optical disk and the rewritable optical disk as described above, information is added by having, in advance at the time of manufacturing, tracks wobbled and modulating the wobbled shapes (refer to Patent Document 1, for example). Moreover, a phase-modulation method is used in a DVD+R and DVD+RW (below also called “DVD+ type” for the sake of convenience).
Furthermore, in a DVD+ type-compliant optical-disk apparatus, for instance, when accessing in an optical disk, a wobble signal corresponding to a wobble shape is detected from returned beam flux output from a light-beam source and reflected at a track, a reference-clock signal is generated from the wobble signal (refer to Patent Document 2), and the wobble signal is phase-demodulated by synchronizing with the reference clock signal for obtaining the information as described above. Moreover, in a DVD+ type, an especially important item of information as information as described above having been added to the track is address information. In the optical-disk apparatus, when recording the user data, the recording position is controlled based on the address information and the reference-clock signal. Thus, if the address information cannot be detected accurately, recording of the user data in a predetermined area is made impossible, possibly causing a recording error. More specifically, in a recordable-type DVD+R, causing a recording error leads to not being able to reuse that optical disk. Therefore, it is very important to accurately detect the wobble signal with high accuracy.
While the wobble signal as described above is detected based on a beam reflected from a track, the reflected beam contains intricate noise components with the wobble signal due to fluctuations, etc. in the data recorded in the optical disk and in the laser-beam output. Thus, an apparatus is proposed such that, for example, a light beam reflected from a track is photo-detected at a photo-detecting device divided into two parts by a dividing line in the direction corresponding to the tangential direction of the track (a two-part divided photo-detecting device), predetermined-level adjustments are made to the output signals (photo-electric converting signals) of each of the photo-detecting devices, and then a difference signal of the signals, or a push-pull signal, is generated and a wobble signal is detected (refer to Patent Document 3 through 7).
Patent Document 1
JP10-069646A.
Patent Document 2
JP2001-035090A.
Patent Document 3
JP2001-266486A.
Patent Document 4
JP2002-117536A.
Patent Document 5
JP2003-059056A.
Patent Document 6
JP2003-077130A.
Patent Document 7
JP8-194969A.
Now, the signal characteristics of the signals output from the photo-detecting devices largely differ between a case in which the RF-signal component is contained within the beam reflected from the track and a case in which the RF-signal component is not contained.
Thus, in order to generate a push-pull signal having a low noise level for both the recording time and the reproducing time, a circuit for generating push-pull signals in a case such that the RF-signal component is contained within the beam reflected from the track and a circuit for generating a push-pull signal in a case such that the RF-signal component is not contained within the beam reflected from the track are needed, resulting in one of the obstacles to reducing the size of the optical-disk apparatus.
Moreover, in order to generate a push-pull signal having a low noise level for both the recording time and the reproducing time, a signal-adjusting circuit for use in a case such that the RF-signal component is contained within the beam reflected from the track and a signal-adjusting circuit for use in a case such that the RF-signal component is not contained within the beam reflected from the track are needed, resulting in another one of the obstacles to reducing the size and the cost of the optical-disk apparatus.